Version 0.2
  * All tests pass with Python 2.5.
  * ComputedCell is now thread safe.
  * ComputedDict is now a little more friendly for subclassing.
  * The 'restrictions' functionality has been moved into it's own set of classes.
  * InputCellDescriptor will now take a default value.
  * Cell descriptors in general are easier to subclass.
  * DependantCell.dependency_changed now takes the dependency as an argument.
  * dependant_cell_stack changed to get_dependant_stack()

Version 0.1.2
  * Python 2.3 is now (mostly) supported.

Version 0.1.1
  * Fixed an oversight with Observer not removing old dependencies.
  * Expanded/Fixed CellDict functionality. (And added some testcases.)

Version 0.1
  * Initial release.
